simple grammar question

[Nola]

I hope the members of this group will excuse my intrusion with my simple grammar question. I have asked two kind and helpful people but still can't really understand this, and hope that among you scholars there will be somebody who can explain this in a way that my thick head can comprehend-
It is about  есть\нет, and быть in the past and future. 
У Саши бЫЛА машина. Sasha had a car.

У Саши не было машины. Sasha didn't have a car.

Ok-I understand the explanation in this case about было \была. In the first sentence, Sasha had a car and it's positive so Sasha is genitive because of the У but HAD is была, matching gender of машина. And in second sentence, Sasha is in genitive case again, car is now genitive because of negative existence,  but было is neuter, because when the existence of something in the past is negative, you use the neuter form of быть.

But see this-------

 Ольга не была сегодня в магазине. Olga was not in the shop today

Тогда  её не было в Москве. At that time, she wasn't in Moscow.

OKAY! What gives here?Both times Olga simply wasn't at a place...but in one case  быть matches Olga's gender and in the next it's neuter...I cannot see what would be the reason for this. 

Any thoughts will be greatly appreciated.

Nola - thick headed student










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